1. Field of the Invention
This invention relates to a semiconductor device in which a semiconductor element is mounted on a supporter of silicon carbide SiC.
2. Description of the Prior Art
In a semiconductor device, the semiconductor element generates heat, and, in order to prevent the instability of characteristics and the accelerated deterioration of effective life of the semiconductor device due to the generation of the heat, it is necessary to provide means for preventing the temperature of the element from rising above an allowable limit. The semiconductor element is very often soldered to electrodes or the supporter and therefore a serious problem is the thermal fatigue of the soldered portions due to the repeated temperature changes caused by the generation of heat. The thermal fatigue is caused by the accumulated mechanical stress resulting from the repeated changes in the soldered portions in temperature since the thermal expansion coefficient of the semiconductor element is different from that of the supporter. If worst comes to worst, the developed thermal fatigue of the soldered portions may cause breaks in electric connection or breakage of the element. Therefore, there must also be provided means for avoiding such faults. Further, the supporter may in some cases serve also as an electrode and it is necessary for the supporter not to have too high an electrical resistance in view of the economy of electric power to be consumed.
In most of the conventional semiconductor devices, the supporter is made of copper and the semiconductor element is mounted on the supporter. This structure, since the supporter is made of metal having excellent electrical and thermal conductivity, has an advantage that the generated heat is easily dissipated. Also excessive electric power consumption can be avoided. However, the thermal expansion coefficient (18.times.10.sup.-6 /.degree.C.) of the copper supporter is much greater than the thermal expansion coefficient (.about.3.times.10.sup.-6 /.degree.C.) of the semiconductor element of silicon so that thermal fatigue tends to accumulate in the soldered portion. This tendency increases with the increase in the capacity of semiconductor element. To solve this problem, either an auxiliary supporter of molybdenum (having THC of 5.2.times.10.sup.-6 /.degree.C.) or tungsten (having THC of 4.3.times.10.sup.-6 /.degree.C.) is inserted as a stress relaxer between the copper supporter and the silicon element, or the thermal expansion coefficient of the supporter is made nearly equal to that of the element by some suitable means. However, molybdenum and tungsten in the form of metal is expensive and heavy (specific weight is 10.27 for Mo and 19.3 for W) and therefore the provision of such an auxiliary supporter goes counter to the requirement for the reduction of cost and weight. Also, for the last means of controlling the thermal expansion coefficients of the contiguous members, various technical points such as the control of the composition and the treatment of materials to be alloyed remain to be improved.